GB2142285A

GB2142285A - Pressurized gas filled tendons

Info

Publication number: GB2142285A
Application number: GB08416340A
Authority: GB
Inventors: William H Silcox
Original assignee: Chevron Research and Technology Co; Chevron Research Co
Current assignee: Chevron USA Inc
Priority date: 1983-06-28
Filing date: 1984-06-27
Publication date: 1985-01-16
Also published as: GB2142285B; NO842598L; YU112784A; ES8601468A1; ES533816A0; YU45901B; FR2548368A1; CA1207155A; GB8416340D0; US4521135A

Description

1

SPECIFICATION

Pressurized gas filled tendons This invention relates to tension leg offshore platform tendons and is concerned with pres surized gas filled tendons for detecting leaks, providing buoyancy and resisting corrosion. A change in pressure denotes a structural defici ency. An increase in tendon buoyancy reduces the weight supported by the buoyant struc ture. Corrosion resistance extends the useful life of the tendon.

In deep water, the use of bottom-founded structures for oil well drilling and production operations is cost prohibitive due to the ex pense of fabrication and installation of such large structures. For water depths in excess of 1,000 feet (305 metres), buoyant offshore - structures moored to the sea floor can be used 85 to perform drilling and production operations cost effectively.

As water depth exceeds 1,000 feet (305 metres), the tension leg platform (TLP) con cept can be introduced to perform oil drilling 90 and production operations. A TLP consists of a buoyant offshore structure moored to fixed sea floor anchor points with vertical tension legs; also referred to as tendons. Drilling, producing and processing equipment as well as crew's quarters are contained in or on the buoyant offshore structure.

Tendon designs include both cable and tu bular leg elements, see for example U.S.

Patent 4,285,615 which discloses a mooring apparatus for a structure floating on a body of water, comprising a corrosion resistant cable system, including a multistrand cable, having voids between adjacent strands, and U.S.

Patent 4,226,555, which discloses a mooring system for a tension leg platform, comprising a tension leg, including a plurality of tubular leg elements having threaded connections between adjacent leg elements.

The use of pre-tensioned vertical mooring elements prevents vertical motion but permits lateral motion of the floating structure during the passage of waves. Pretensioning is accom plished by deballasting the buoyant offshore structure after the tendons are connected be tween the buoyant structure and fixed sea floor anchor bases.

Tendon inspection is necessary as both a maintenance expenditure and safety precau tion. Tendon repair and replacement are both very expensive and laborious operations.

Cracks and corrosion due to exposure to sea water decrease the failure load and working lifetime of the tendon. The desirability of minimizing tendon corrosion has been recog nized in the art. The aforementioned U.S.

Patent 4,285,615 discloses an invention for providing a corrosion resistant design for a tension leg cable which isolates the steel wire cable from the sea water environment. 130 GB 2 141 285A 1 The present invention provides an apparatus and method for detecting structural deficien cies in a tubular tendon, increasing its buoy ancy and extending its useful life.

According to one aspect of the invention, there is provided an apparatus for detecting a leak in a tension leg platform tendon, com prising:

a fluid tight tubular tendon; means for supplying gaseous fluid to said tendon; means for pressurizing said gaseous fluid; means for monitoring pressure, said means monitoring variations in gaseous fluid pres- sure in said tendon; means for connecting an upper end of said tendon to a buoyant offshore structure; and means for connecting a lower end of said tendon to an anchor means connected to the sea floor.

According to another aspect of the invention, there is provided a method of detecting a leak in a tension leg offshore platform tendon, comprising the steps of:

connecting a tendon between a buoyant offshore platform and an anchor means connected to the sea floor at an offshore location, said tendon being a fluid tight tubular member; supplying a gaseous fluid to said tendon; pressurizing said gaseous fluid; and monitoring gaseous fluid pressure in said tendon to detect leaks therein.

According to a further aspect of the inven- tion, there is provided a tension leg offshore platform having anchor means connectable to the sea floor; at least one tendon connectable between said platform and said anchor means, said tendon being a fluid tight tubular member; means for supplying gaseous fluid to said tendon; means for pressurizing said gaseous fluid; and means for monitoring pressure, said means being capable of monitoring variations in gaseous fluid pressure in said tendon.

The present invention provides a method and means for detecting leaks in a tubular tendon, increasing its buoyancy and extending its useful life. A plurality of tendon segments, each consisting of a tubular element and sealable couplings, are joined to provide a single elongated tubular tendon. The tubular tendon is filled with gaseous fluid which is preferably a corrosion inhibiting gaseous fluid. A compressor is utilized to pressurize the contents of the tubular tendon and pressure gauges monitor variations in pressure.

The corrosion inhibiting gaseous fluid protects the interior tendon wall from salt water corrosion. The gaseous fluid increases the buoyancy of each tendon, thereby reducing the weight supported by the buoyant offshore structure. Variations in pressure indicate 2 cracks or punctures through the tendon or an inadequate coupling seal.

The object of the present invention is to provide a method and means for detecting leaks in a tubular tendon, increasing its buoyancy and extending its useful life. A method and means for detecting leaks indicating structural deficiencies promotes safety and reduces routine maintenance expenditures. In- creasing the buoyancy of the tendon reduces the weight supported by the buoyant offshore structure; permitting a more effecient design. Increased tendon life provides more cost effective deep water drilling by reducing maintenance, repair and replacement of the tendons.

Another object of the present invention is to provide an improved design for a tension leg platform incorporating the invention described herein.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made to the accompanying drawing in which:

Figure 1 is an elevation schematic view, partially in section, of a tension leg platform; Figure 2 is an enlarged detailed view of part of a tendon of Figure 1; and Figure 3 is a section of the tendon of Figure 2 taken along the line 3-3.

Referring now to the drawing, Figure 1 shows an elevation schematic vievipartially in section, of a tension leg platform (TLP) 1 deployed at an offshore drilling site. A lower platform 2 is provided on which may be mounted crew's living quarters, well test equipment and processing equipment. An upper platform 3 is provided on which may be mounted a pilot house, cranes, the drilling derrick, skid base, the drill string and a heli- copter landing site. Similar conveniences as are known to those skilled in the art of oil exploration and production may also be stored on the lower and upper platforms. Platforms 2 and 3 are supported by a plurality of annular support columns 4. When the TLP is in its illustrated buoyant condition, columns 4 and pontoons 5 extend beneath the surface of the water. A plurality of tendons 6 extend from each support column 4 to anchor means con- sisting of a foundation template 7 secured to the sea floor 8 with friction piles 9, thereby restricting movement of the structure. A drill string 10 and risers 11 extend from platform 1 or 2 between pontoons 5 to the see floor 8 during drilling and producing operations. Well template 12 maintains the risers in a stationary position relative to the sea floor 8.

Referring to Figure 2, an enlarged detailed view of a part of tendon 6 depicts the tendon as a tubular element. A plurality of tendon segments, each consisting of a tubular element and sealable couplings, are joined to provide a single elongated tubular tendon. The tubular element typically has a relatively thin wall compared to its overall diameter. A GB 2 141 285A 2 tubular element has been designed utilizing inside and outside diameters of 18 inches (45.7 cm) and 20 inches (50.8 cm), respectively. Figure 3 shows a section view of the tendon of Figure 2 taken along the line 3-3.

In accordance with the present invention, corrosion inhibiting gaseous fluid enters the tendon through a conduit located at its upper end. The gaseous fluid may be an inert gas, preferably nitrogen. Alternatively air may be used.

Subsequent to the introduction of gaseous fluid to the tendon, compressor 13 supplies pressure through the conduit to the tendon's contents. A pressure in excess of the maximum hydrostatic pressure exerted by the sea water on the tendon is recommended to avoid the instance where the pressure inside the tendon is equal to the sea water pressure at the same elevation. A positive net internal pressure is utilized to detect a leak. A valve is closed to retain the pressurized contents. Pressure gauges monitor the pressure therein. Reductions in pressure, in excess of a predet- ermined value, activate a signal to inform crew members of a deficient tendon.

The corrosion inhibiting gaseous fluid protects the interior walls of the tubular tendon from exposure to sea water. The gaseous fluid provides buoyancy and reduces the tendon weight supported by the offshore buoyant structure. Barring any pressurizing malfunctions, a change in pressure indicates a leak in the tendon attributable to a crack or puncture through the tendon or an inadequate coupling seal.

Variations in the embodiment described with reference to the drawing will be readily apparent to those skilled in the art.

Claims

1. Apparatus for detecting a leak in a tension leg platform tendon, comprising:

2. Apparatus as claimed in Claim 1, wherein said gaseous fluid is corrosion inhibiting.

3. Apparatus as claimed in Claim 2, wherein said gaseous fluid is an inert gas.

4. Apparatus as claimed in Claim 3, wherein said gaseous fluid is nitrogen.

5. Apparatus as claimed in Claim 1, wherein said gaseous fluid is air.

6. Apparatus as claimed in any preceding 3 GB 2 141 285A 3 claim, wherein in use the gaseous fluid pressure in said tendon is in excess of the maximum hydrostatic pressure exerted on said tendon by the sea water in an offshore location.

7. A tension leg offshore platform having anchor means connectable to the sea floor; at least one tendon connectable between said platform and said anchor means, said tendon being a fluid tight tubular member; means for supplying gaseous fluid to said tendon; means for pressurizing said gaseous fluid; and means for monitoring pressure, said means being capable of monitoring variations in gaseous fluid pressure in said tendon.

8. A tension leg platform as claimed in Claim 7, wherein said gaseous fluid is corro sion inhibiting.

9. A tension leg platform as claimed in Claim 8, wherein said gaseous fluid is an inert gas.

10. A tension leg platform as claimed in Claim 9, wherein said gaseous fluid is nitrogen.

11. A tension leg platform as claimed in Claim 7, wherein said gaseous fluid is air.

12. A tension leg platform as claimed in any one of Claims 7 to 11, wherein in use the gaseous fluid pressure in said tendon is in excess of the maximum hydrostatic pressure exerted on said tendon by the sea water in an offshore location.

13. A method of detecting a leak in a tension leg offshore platform tendon, comprising the steps of:

connecting a tendon between a buoyant offshore platform and an anchor means con- nected to the sea floor at an offshore location, said tendon being a fluid tight tubular member; supplying a gaseous fluid to said tendon; pressurizing said gaseous fluid; and monitoring gaseous fluid pressure in said tendon to detect leaks therein.

14. A method according to Claim 13, wherein said gaseous fluid is corrosion inhibiting.

15. A method according to Claim 14, wherein said gaseous fluid is an inert gas.

16. A method according to Claim 15, wherein said gaseous fluid is nitrogen.

17. A method according to Claim 13, wherein said gaseous fluid is air.

18. A method according to any one of Claims 13 to 17, wherein said gaseous fluid maintains a pressure in excess of the maximum hydrostatic pressure exerted on said tendon by the sea water at said offshore location.

19. A tension leg offshore platform substantially as hereinbefore described with reference to, and as shown in, the accompanying draw- ing.

20. A method of detecting a leak in a tension leg offshore platform tendon, substw:tially as hereinbefore described with reference to the accompanying drawing.

Printed in the United Kingdom for Her Majesty's Stationery Office, Dd 8818935, 1985, 4235Published at The Patent Office, 25 Southampton Buildings. London, WC2A lAY, from which copies may be obtained.